copy.h: move declarations for copy.c functions from cache.h

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/difftool.c b/builtin/difftool.c
index 3ffb052..4095079 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -15,6 +15,7 @@
 #include "cache.h"
 #include "abspath.h"
 #include "config.h"
+#include "copy.h"
 #include "builtin.h"
 #include "run-command.h"
 #include "environment.h"